Since the discovery of oxide superconducting materials having superconductivity at a temperature of liquid nitrogen, superconducting wire materials aimed at applications to electric power devices such as cables, current limiting devices, and magnets have been actively developed.
Long superconducting wire materials are required to produce, for example, superconducting cables and superconducting coils for superconducting apparatuses. Therefore, a plurality of superconducting wire materials are sequentially connected to each other to achieve lengthening of superconducting wire materials (e.g., refer to PTL 1 and PTL 2).
However, oxide superconducting materials for forming superconducting films of superconducting wire materials have a stable phase up to a temperature close to its melting point, but are easily decomposed at a temperature higher than its melting point. Therefore, when a plurality of superconducting wire materials are joined to each other via superconducting film surfaces, a method such as thermal diffusion treatment that is typically used for joining metals cannot be employed.
Thus, methods for joining protective layers or stabilizing layers formed on superconducting films to each other by diffusion joining with Ag or by using solder have been generally employed. However, when such a method is employed, finite resistance is generated and the joining is not achieved in a superconducting state, which poses a problem in that superconducting wire materials cannot be used in a permanent current mode.
Accordingly, the present inventors have developed a joining technique in which a superconducting layer made of an oxide superconducting material is formed on a joining surface, and superconducting film surfaces of superconducting wire materials are joined to each other via the joining surfaces (refer to PTL 3). Thus, joining can be performed in a superconducting state with zero resistance.
Specifically, this joining technique uses a metal organic deposition (MOD) method. A solution (MOD solution) containing an organic compound of a metal constituting an oxide superconducting material is applied onto the joining surface of a superconducting wire material, and temporary firing thermal treatment is performed to form a temporarily fired film serving as a precursor of the oxide superconducting material. By performing main firing thermal treatment while the temporarily fired films are pasted to each other, a superconducting layer of the oxide superconducting material is formed as a joining layer between superconducting films of two superconducting wire materials, thereby joining superconducting film surfaces to each other.